This invention is broadly concerned with improving the performance of implantable cardiac defibrillators. These are products used for treatment of tachyarrhythmias, the irregular, rapid heartbeats that can be fatal if uncorrected. These devices produce a shock to the heart when called upon to correct an onset of tachyarrhythmia. The shock is delivered by a capacitor, which has been charged by the battery.
The electrical power source of choice at present for operating these electronic devices has been the Li/SVO battery or cell where SVO stands for silver vanadium oxide of the type disclosed in U.S. Pat. No. 4,310,609 or 4,391,729 issued to Liang et al or U.S. Pat. No. 5,221,453 issued to Crespi which are incorporated herein by reference in their entirety and used in batteries or cells as disclosed in U.S. Pat. Nos. 5,312,458; 5,298,349; 5,250,373; 5,147,737; 5,114,811; 5,114,810; 4,964,877; and 4,830,840. This particular cell chemistry has been useful for the defibrillator application because of its ability to produce pulses of energy which can charge the capacitors of the defibrillator in a timely manner. For example, once the implantable defibrillator has identified a tachyarrythmia the shock must be provided to the patient in a matter of a few seconds. Thus, the Li/SVO battery is typically called upon to charge the capacitors to deliver a shock of up to 40 joules within 10 seconds or less and to do so several times in succession if additional shocks are required. Unfortunately, on long-term discharge, these cells can develop high resistance that impairs their ability to charge the capacitors in a timely manner and therefore renders much of the capacity of the cell unavailable for long term use in an implantable defibrillator. Further, the end of service (EOS) determination in these cells is complicated by the variable nature of the resistance buildup.
This invention then is more specifically directed to improvements in Li/SVO cells to avoid this resistance buildup and render their operation more predictable as a basis for simple EOS determination.